Because of their high water content, mouthwashes present particular challenges in preventing microbial contamination. Dual phase mouthwashes present particular challenges in that the hydrophilic and hydrophobic phases should remain separated and form a temporary emulsion when mixed. The emulsion spontaneously reverts back to the two original phases after rest, without the formation of an emulsion. See, e.g., U.S. Patent Publication 20090311200, the contents of which are incorporated herein by reference. Selection of a preservative which is both effective and which does not impair the physical properties of the dual phase formulation is not trivial. Additionally, some preservatives negatively affect the taste or aesthetics of the product. Finally, conventional agents such as ethanol and paraben preservatives may be undesirable for certain indications or in particular markets.
Accordingly, there is a need to identify improved preservative agents for use in dual phase mouthwashes.